This invention relates in general to pressure transducers. More specifically, it relates to a variable capacitance type sensor with a movable electrode center-mounted on an edge-mounted diaphragm.
A wide variety of transducers are known which convert fluid pressure or force into movement of a diaphragm, which in turn is converted into an electrical signal that corresponds to and measures the pressure or force. One type of transducer which has been proven useful for high accuracy and ruggedness for a wide range of applied pressures measures the diaphragm motion capacitively. The diaphragm forms one plate of a variable gap capacitor. One or more electrodes form the other plate or plates.
U.S. Pat. No. 3,859,575 (Reexamination Certificate B1 No. 3,859,575, May 3, 1988) issued to the present applicant and a co-inventor describes a successful sensor for a capacitive transducer, one where an electrode plate is mounted at its center to the center of the diaphragm. This arrangement enhances the change in capacitance for a given movement of the diaphragm, which increases the accuracy of the transducer. This arrangement also improves the thermal response and avoids capacitance shifts due to thermal stress. It has short thermal paths that provide a fast response to changes in temperature. These advantages were achieved while at the same time meeting other important design objectives such as resistance to mechanical shock and vibration and low hysteresis.
While the '575 sensor has proven successful, it has not been effective in meeting the need for an accurate, rugged and reliable sensor that also has a sufficiently low cost of manufacture that can be used in high volume applications such as in automobiles, off-road vehicles, compressors and many other industrial applications. A principal drawback is the cost of machining the diaphragm and other components such as electrode installation material. In the FIGS. 1, 3 and 4 embodiments shown in the '575 patent, the fitting and diaphragm are bored and threaded, a boss is machined on the diaphragm for at least high pressure applications, and an anti-hysteresis groove is machined in the diaphragm. The boring and threading of the diaphragm is particularly important since it holds a threaded stud that locates a position shim, the electrode, and a nut to secure this assembly. This level of precision, numerically controlled machining is much too costly for high volume, low-cost applications. Besides these machining costs, these embodiments require that the electrode be constructed as an assembly having an internal dielectric ring to provide electrical insulation between the diaphragm and the electrode. In addition, the shim is a permanent spacing element. It is tightened against the diaphragm and the electrode assembly. This produces a high level of friction as the diaphragm deforms, which in turn produces hysteresis. The anti-hysteresis groove controls the hysteresis, but the groove necessitates a diaphragm of a certain thickness and requires a further machining operation to produce the groove. For low pressure applications, machining a very thin diaphragm to a given thickness and to be sufficiently flat proved to be costly.
FIG. 2 of the '575 patent shows an early attempt at a low cost version of the center mount design. A stamped metal plate diaphragm has a central flat portion with its edges clamped to the end of a fitting and sealed with an o-ring. A mounting post is spot welded to the center of the flat diaphragm portion. The electrode is an integral, metal-only piece with a flat, annular outer portion and a central cylindrical section surrounding the post. An epoxy resin bridges the central post and the cylindrical section. It provides mechanical support for the electrode as well as electrical isolation.
While this arrangement avoids the machining costs of the FIGS. 1, 3 and 4 embodiments, it has other significant drawbacks which have prevented its use commercially. For high pressure applications, e.g. to 5,000 to 10,000 lbs/in.sup.2, a thick diaphragm is necessary to withstand the substantial accumulated fluid force. During deflection stress in the diaphragm concentrates at its center and its periphery. After repeated cycles of operation, the high stresses at these points can deteriorate the repeatability of the movement, or cause the construction to fail, as by breakage of the post-to-diaphragm weld or fatiguing of the clamped metal edge at the periphery. For low pressure applications, the diaphragm must be thin in order to follow the pressure changes. With the FIG. 2 diaphragm, this thin diaphragm offers little resistance to a sideways rotation of the post and the electrode mass assembled to it. This mechanical spring-mass system has a low natural frequency; it is highly susceptible to shock and vibration.
Another serious deficiency is that the adhesive used to assemble the electrode to the post is also the source of electrical insulation between the plates of the variable capacitor. The FIG. 2 construction uses temporary shims. The cement is applied and the shims removed when it sets. Suitable dielectric adhesives are organic materials such as the epoxy resin specified in the '575 patent. But epoxy resin and other organic cements are not high quality insulators. Their dielectric constants are not stable over time or stable with changes in atmospherics such as ambient temperature and humidity. This is a serious problem since the capacitance of the epoxy-filled gap between the post and electrode is in parallel with the variable capacitance gap making the measurement. Changes in the epoxy dielectric therefore produce errors in the measured pressure. While a material such as glass is much more stable, it melts at such a high temperature that it is impractical to use as a cement during assembly.
It is therefore a principal object of the invention to provide a variable, center-mounted capacitive sensor having an extremely low cost of manufacture, while retaining the good performance characteristics of the center-mounted design characteristic of more expensive, highly machined parts.
Another principal advantage is to provide a sensor and method of sensing that is rugged with respect to resistance to shock, vibration, atmospherics such as humidity and temperature changes, both ambient and sudden, and drift over time.
Another advantage is to provide the foregoing advantages without the use of a permanent shim.
Still another advantage is to provide a sensor and method of use which is highly resistant to material fatigue.
Still another object is to provide the foregoing advantages over a full range of pressures, from very low to very high.
Yet another advantage is to provide all of the foregoing advantages while requiring a low skill level for manufacture and assembly.